Accordingly, the present application describes the discovery of a late or adult onset form of peroxisomal dysfunction in humans. The disease manifests in subjects of all ages but the incidence increases with increasing age after age 50 and peaks in 60-69 year olds and decreases thereafter. Subjects suffering from age related plasmalogen deficiencies have abnormally low levels of circulating plasmalogens in their serum and an increased prevalence of colon cancer, prostate cancer, lung cancer, breast cancer, ovary cancer, kidney cancer, cognitive impairment and dementia relative to subjects without age related plasmalogen deficiencies.
The biosynthesis of plasmalogens has been recently reviewed in detail1. The first two steps in the plasmalogen biosynthesis pathway are carried out exclusively in peroxisomes (see FIGS. 1 and 22 for the full pathway). The free hydroxyl group of dihydroxyacetone phosphate (DHAP) is first acetylated by DHAP acyltransferase (DHAP-AT). The ether bond (plasmanyl) is then created by replacing the sn-1 acyl group with a fatty alcohol by alkyl-DHAP synthase. Loss of function of either of these two enzymes through point mutations, impaired peroxisomal targeting or due to general peroxisomal dysfunction results in a severe plasmalogen deficiency. The remaining key synthetic processes occur in the endoplasmic reticulum (ER) where the sn-2 position is acylated and phosphoethanolamine is added to the sn-3 position to create plasmanyl glycerophosphoethanolamine (GPE). The final step involves a plasmanyl-specific enzyme that desaturates the 1-O-alkyl ether to form the vinyl ether (plasmenyl) GPE species, commonly referred to as PlsEtn or plasmenylethanolamine, also commonly known as ethanolamine plasmalogens. All cells in the body are capable of synthesizing these molecules.
Peroxisomes were first discovered in the late 1960's by de Duve2. Since that time over 50 different biochemical pathways have been described to be performed by peroxisomes3. The nine primary biochemical systems are listed in Table 1. The first peroxisomal disease of humans (cerebro-hepato-renal syndrome of Zellweger) was clinically described in 1964 by Bowen et al4. In 1973, it was discovered that these patients had disturbed mitochondrial function and no functional peroxisomes5. Currently, there are seventeen human diseases that are characterized as peroxisomal in origin (reviewed by Wanders6, Table 2). The peroxisomal disorders are not uniform. Different disorders present vastly different biochemical abnormalities with some of these abnormalities overlapping. Accordingly, the diseases can be characterized as being either a disorder of peroxisomal biogenesis in which there is a general, overall peroxisomal deficit or a disorder of a particular peroxisomal protein or enzyme system (Table 3).
Of the disorders currently characterized, only Rhizomelic Chondrodysplasia Punctata (RCDP) can be said to be caused by plasmalogen deficiency alone. The RDCP disorders are further grouped into type I, II, or III. All three RDCP types exhibit decreased levels of plasmalogens in the plasma and a decreased de novo synthesis capacity of plasmalogens in the liver. Peroxisomal function in these subjects is believed to be otherwise normal, except for decreased α-oxidation of phytanic acid in type I RCDP.
Subject suffering from RCDP exhibit severe mental retardation and dysplasias of the bone which result in stunted growth among other abnormalities. Subjects with RCDP show numerous neurological abnormalities, the most striking of which is delayed myelination7,8, which is believed to be a direct result of decreased plasmalogen synthesis. Most subjects with RCDP do not live more than two years from birth.
Dysplasia is an abnormality in the appearance of cells indicative of an early step towards transformation into a neoplasia. Dysplasia is a pre-neoplastic state of a cell. This abnormal growth is restricted to the originating system or location, for example, a dysplasia in the epithelial layer will not invade into the deeper tissue, or a dysplasia solely in a red blood cell line (refractory anaemia) will stay within the bone marrow and cardiovascular systems. The best known form of dysplasia is the precursor lesions to cervical cancer, called cervical intraepithelial neoplasia (CIN). This lesion is usually caused by an infection with the human papilloma virus (HPV). Dysplasia of the cervix is almost always unsuspected by the woman. It is usually discovered by a screening test, the pap smear. The purpose of this test is to diagnose the disease early, while it is still in the dysplasia phase and easy to cure. Dysplasia is the earliest form of pre-cancerous lesion in which a cell begins to change away from its normal form to an abnormal, less differentiated form. Carcinoma in situ, meaning ‘cancer in place’, represents a final transformation of a dysplasic cell to cancer, though the cancer remains local and has not moved out of the original site. Dysplasia is not cancer.
Cancer is a state where the cells have lost their tissue identity and have reverted back to a primitive cell form that grows rapidly and without regulation. Invasive carcinoma is the final step in this sequence. It is a cancer which has invaded beyond the original tissue layer and is also able to spread to other parts of the body (metastasize), starting growth of the cancer there and destroying the affected organs. It can be treated, but not always successfully. However, if left untreated it is almost always fatal.
In summary, a selective deficiency in plasmalogen biosynthesis is known to result in the clinical manifestation of severe neurological and cellular growth abnormalities. Furthermore, the survival disadvantage of decreased plasmalogen biosynthesis is severe.
It is well known that many diverse human diseases such as cancer, dementia, or decreased cognitive functioning increase in incidence with age. From an epidemiological and statistical perspective, these diseases often look very similar. However, from a clinical perspective, each of the cancers, dementias, and decreased cognitive functioning are very different. Currently, the largest risk factor for these disorders is the subject's age. Furthermore, it is well established that most cancers, dementias, and decreased cognitive functioning have a long prodromal phase (5-15 years) in which the disease is present but at a sub-clinical manifestation. There are few, if any, practical methods to accurately and precisely identify subjects with a clearly elevated risk. Accordingly, there is a tremendous need to be able to accurately identify subsets of the general population subjects with biochemical abnormalities that are causally linked to the known biochemical etiology of chronic age-related disorders such as cancers, dementias and decreased cognitive functioning and then treat these subjects with safe and well tolerated therapeutics that can correct the biochemical abnormality and reduce the risk of disease occurrence in this sub-population.
Impaired membrane cholesterol regulation, membrane dynamics, muscarinic receptor signal transduction, and APP processing are implicated to various degrees and to various symptoms and pathologies observed in dementia and decreased cognitive functioning. The association of these biochemical systems and dementia are well established. Acetylcholinesterase (ACE) inhibitors act by increasing the retention time of ACh in the synaptic cleft and therefore increase muscarinic receptor transduction9. Statins act by reducing cholesterol levels10. Amyloid lowering drugs are currently in development and clinical trials for removing the accumulation or to reduce the production of amyloid plaques. Therefore, the direct findings presented above, strongly implicate plasmalogen biosynthesis impairment in the etiology of dementia and cognitive impairment. In applicant's co-pending application PCT/CA2007/000313, which is published as PCT Publication No. WO 2007/098585, metabolites selected from phosphatidlycholine-related compounds, ethanolamine plasmalogens, endogenous fatty acids, essential fatty acids, lipid oxidations byproducts, and metabolite derivatives of these metabolic classes were found to be at lower levels in samples from patients suffering from dementia. In the present invention a decrease in plasmalogens has been found in patients suffering from other age related diseases.
One of the defining features regarding cancer cells is that, unlike normal cells which rely almost entirely upon respiration for energy, cancer cells can utilize both respiration and glycolysis for energy. In cancer, much work is now focused on developing drugs that inhibit the glycolysis pathway. One of the defining features of aerobic glycolysis in cancer is an enhanced mitochondrial citrate export and the use of cytosolic citrate to form acetyl-CoA. Therefore, the direct findings presented above, that an impairment in plasmalogen biosynthesis result in both increased membrane cholesterol levels and increased cytosolic acetyl-CoA utilization, strongly implicate plasmalogen biosynthesis impairment in cancer etiology.
The present application describes a subset of adult humans (>age 40) who have abnormally low levels of plasmalogens in their serum. This deficiency has been determined to be due to decreased plasmalogen synthesis and not due to increased oxidative stress. Subjects with this disorder have an increased prevalence of cognitive impairment, dementia and cancer. The early diagnosis of these diseases, or the assessment of risk in subjects before they get these diseases will result in a tremendous improvement on the long-term quality of life of these subjects as well as have a tremendous long-term cost saving to existing health care systems.